The file extension is the least interesting thing about your video
A video file is two things stacked together. The container (.mov, .mkv, .avi, .webm) is the box: it holds the streams, the timing, the chapters, the subtitle tracks. The codec is what is actually in the box, the compressed pictures themselves, and it is almost always one of a short list: H.264, HEVC, VP8, VP9, or AV1. Two files with the same extension can carry completely different codecs, and two files with different extensions can be byte-for-byte the same video underneath.
That is why this converter does not keep a per-extension list of what it accepts. It hands your file to the decoder your browser already ships, the same one that plays every video you watch, and asks a single question: can you play this? If the preview in the converter moves, every control that follows will work, whatever the extension says. If it stays black, the codec inside is one your browser has never learned, and the fix is about the codec, not the name on the box.
Container by container: what usually opens and what usually does not
| Container | Where it usually comes from | What is usually inside | Opens in a browser? |
|---|---|---|---|
| MP4 | Phones, editors, almost everything | H.264, sometimes HEVC | Yes. The default; see the MP4 page |
| MOV (QuickTime) | iPhone camera, Mac screen recordings, Final Cut | H.264 or HEVC; ProRes from pro exports | H.264 yes, HEVC depends on the browser, ProRes no |
| M4V | Apple app exports, old iTunes video | H.264 or HEVC in an MP4 box | Yes, unless it is FairPlay-locked store video |
| WebM | OBS, Chrome capture, MediaRecorder | VP8, VP9, or AV1 | Yes, in every current browser |
| MKV (Matroska) | OBS sessions, remuxes, archives | Anything: H.264, HEVC, VP9, AV1 | Usually, when the video track is H.264 or VP9 |
| AVI | Camcorder transfers, capture cards, 2000s downloads | Motion JPEG, DivX, Xvid, older codecs | Sometimes. Old codecs often go black |
| WMV | Movie Maker, old PowerPoint recordings | Microsoft's own Windows Media codecs | Rarely. Plan on the VLC pass |
| FLV | Flash-era downloads and tutorials | Often H.264, sometimes Sorenson or VP6 | Patchy. H.264 inside helps |
| TS / M2TS | DVRs, broadcast captures, HLS segments | Usually H.264 | Inconsistent across browsers |
The black preview, and the two-minute rescue
When a file loads but the preview sits black, the browser read the container fine and then met a codec it cannot decode. There is no setting on this side that changes that verdict. What changes it is rewrapping the footage in something every browser speaks, and the free tool for that is VLC: open the file, choose Convert / Save, pick an H.264 MP4 profile, and drop the new copy in. Your original is untouched.
Three cases deserve a specific note. ProRes .mov files, the kind Final Cut exports and iPhone Pro models can shoot, are heavy on purpose (a minute of 1080p ProRes 422 HQ runs well past a gigabyte) and no browser ships a decoder for them; export an H.264 copy from the app that made it. HEVC support varies by browser and operating system, so a clip that opens in Safari on a Mac may go black in Firefox on Windows; setting the phone camera to Most Compatible avoids it at the source. And FairPlay-protected M4V from an old store purchase is locked by design. No converter can open it, and none should claim to.
What survives the trip into a GIF, and what is left at the door
GIF is a 1987 image format with an animation loop bolted on, so most of what a modern container carries has nowhere to go:
- Audio is dropped. GIFs are mute. If the sound matters, export a silent MP4 here and add audio elsewhere, or keep the source.
- Subtitle tracks and chapters are ignored. MKV and MP4 can carry several of each; the GIF gets the picture only. Burn a line of text in with a caption instead.
- Transparency is flattened. A WebM with an alpha channel comes out opaque, see-through regions landing on a solid background.
- Variable frame rate gets evened out. Screen recorders and MediaRecorder write uneven timing. You pick one steady output rate, and 12 to 15 fps smooths a lumpy capture.
- Color gets budgeted. Each frame holds at most 256 colors. The encoder builds the palette from the footage itself and dithers the gradients so skies do not band.
Old footage and new footage want different dials
Legacy containers tend to hold small, soft, 4:3 video. A 640 by 480 AVI from a camcorder transfer or a 320 by 240 FLV does not need downscaling at all, rarely rewards more than 10 to 15 fps, and often looks identical at 128 colors. Lock the crop to 4:3 to keep the original shape instead of stretching it to widescreen.
Modern containers usually hold the opposite: 1080p or 4K, 30 or 60 fps, from a phone or a capture of a high-density screen. The size problem there is pixels, not codec, so bring the width down to 480 to 800 pixels before anything else and watch the live size estimate fall. The small-GIF guide ranks every lever by how much it saves, and the high-quality page covers the other direction, when fidelity beats file size.
Your archive stays in your archive
The files that end up in odd containers are often the personal ones: family video in an AVI, a DVR recording in a .ts, a training capture in a WMV from a laptop that no longer exists. None of it is sent anywhere here. The container is opened, decoded, and re-encoded in the tab on your own machine, which is also why there is no server quota on input size; the practical ceiling is your memory, with a guard at roughly 500 MB per file. The no-upload page explains how the site enforces that, down to the security header that blocks the page from sending data out.